1. Field of the Invention
The present invention relates to a transmission liquid crystal display apparatus and, more particularly, to a transmission color liquid crystal display apparatus in which a color filter is arranged at the display surface side of a liquid crystal cell and a light source is arranged at its rear surface side.
2. Description of the Related Art
A liquid crystal display apparatus is widely used as a flat type, low-power consumption display. Especially in recent years, significant technical developments have been made to increase the number of pixel segments and the size of a screen. Therefore, for a binary-leveled display, an apparatus having 640.times.400 pixels and a diagonal of 10 inches or more is now available. In addition, a liquid crystal TV capable of halftone display by using an active matrix type liquid crystal is put into practical use. The active matrix system in which crosstalk does not occur and the number of pixel segments can be easily increased is expected to be used as a future large screen.
Liquid crystals driven by the active matrix system are mostly of a twisted nematic (TN) type and then of a guest host (GH) type. The TN type has advantages such as a high liquid crystal resistance, a high pixel segment voltage holding rate, and a high contrast ratio of 30 to 100 or more. The TN type, however, has a problem of a narrow angle of view (especially in a specific direction). The GH type has a wider angle of view than that of the TN type but inferior to the TN type in other factors. In addition, in the GH type, a dye is decolored upon light radiation over a long time period to reduce a contrast.
A narrow angle of view of the TN type liquid crystal display apparatus is problematic especially in a halftone display in which liquid crystal molecules are obliquely aligned. This problem appears as a phenomenon in which if a screen size is increased, a contrast differs between upper and lower portions or right and left portions even with the same voltage. That is, image quality is degraded when the screen size is increased. In addition, light transmittance dependency with respect to a pixel segment voltage differs between the normal direction and the oblique direction on the screen. Therefore, in a color display in which a color filter is provided for each pixel segment, a color tone differs in the normal and oblique directions. As a result, image information undesirable differs in accordance with the viewing direction.
FIG. 1 shows an arrangement of a conventional transmission color liquid crystal display apparatus. Referring to FIG. 1, reference numeral 61 denotes an active matrix substrate; 62, a polarizing plate; 63, a pixel segment electrode; 64, a counter electrode; 65, a TN type liquid crystal layer; 66, a color filter layer; 67, a glass plate; 68, a polarizing plate; 69, a light source; and 70, a light diffusion plate arranged in front of the light source 69.
This conventional display apparatus has the following problem concerning an angle of view. Light rays from the light source 69 are diffused in all directions by the diffusion plate 70. As a result, the light rays from a liquid crystal cell can be observed in the normal direction and a direction oblique with respect to the display surface. In FIG. 1, assume that light rays transmitted through a certain pixel segment corresponding to a certain color in the normal direction are light rays A and those in the oblique direction are light rays B, and light rays transmitted through a pixel segment adjacent to the above pixel segment and corresponding to another color in the normal direction are light rays A' and those in the oblique direction are light rays' B. When a voltage of the latter pixel segment is set in a light-shut state, i.e., when a shutting voltage is applied to the pixel segment electrode 63, the intensity of the light rays A' is low and that of the light rays A is high in the normal direction. Therefore, the rays can be viewed on the screen as chromaticity at a point A corresponding to the light rays A in a chromaticity diagram shown in FIG. 2. In the oblique direction, however, the intensity of the light rays B' is higher than that of the light rays A' since the angle of view of the liquid crystal is narrow. Therefore, the light rays B' are mixed and viewed together with the light rays B' i.e., the adjacent light rays having different colors are mixed. As a result, the rays are moved in a white direction (arrow direction) in the chromaticity diagram in FIG. 2 and viewed as a color close to white on the screen. For this reason, image quality of a flat type TV using the conventional liquid crystal is inferior to that of a CRT. Therefore, applications of such a flat type TV are limited to hobbies such as a pocket TV.
As described above, it is difficult to obtain a large screen by using the conventional transmission liquid crystal display apparatus since the angle of view of the liquid crystal is narrow. Especially in a color display, image quality in a halftone display is poor.